Rollover vent valves are well known in the art for venting fuel vapor from a vehicle fuel tank to the atmosphere or, more recently, to a vapor recovery system such as a carbon canister. Such valves typically vent the interior of the fuel tank to the carbon canister when the fuel in the tank is below the level of the valve, and are closed by a float when the fuel surges in a rollover situation. Closing the valve in response to surging liquid fuel level prevents the overflow of liquid fuel into the carbon canister. If employed as a fill-control vent valve, rather than a rollover vent valve, closure is in response to a full fuel level rather than surge and results in a pressure head within the fuel tank and filler pipe to operate automatic shutoff apparatus built into the fuel fill nozzle.
In U.S. Pat. No. 4,753,262 to R. Bergsma, issued Jun. 28, 1988, a rollover vent valve having a venting outlet closed by a peel-away valve disc is disclosed. When liquid fuel reaches the valve a float member is raised to force the valve disc against the vent outlet, thereby closing it to prevent fuel from reaching the canister. In order to ensure positive opening of the valve when the fuel level drops, the float and the structure connecting it to the valve plate operate to circumferentially lever or peel the valve plate off the vent outlet.
As noted in the above-cited patent, increasing the rate of vapor flow from the fuel tank through the valve to the canister requires an increase in the area of the valve vent outlet. Providing the valve with a large vent outlet increases the force acting to maintain the valve in the closed position once the fuel level drops; i.e., the greater the area of the vent outlet, the greater the closing force on the valve plate caused by the tank/canister pressure differential. With a large vent outlet and corresponding valve plate subjected to a substantial tank/canister pressure differential, the float can "hang-up" despite a drop in the fuel level. The valve would then remain closed, resulting in a rapid and undesirable buildup of pressure in the unvented tank.